This invention relates to memory and logic circuits and, more particularly, to such circuits employing current-switched Josephson junction devices.
In a paper entitled "Possible New Effects in Superconductive Tunneling," Physics Letters, Vol. 1, p. 251 (1962), B. D. Josephson predicted theoretically that at zero voltage a supercurrent would flow between two superconductors separated by a thin insulating barrier by a mechanism known as two-particle or pair tunneling. Above a critical supercurrent I.sub.J (also referred to as a threshold or zero voltage current) current flows by single particle tunneling at a voltage V = 2.DELTA., where 2.DELTA. is the superconductor gap voltage. This Josephson effect is exhibited by several device structures which entail two-particle tunneling: SIS devices (U.S. Pat. No. 3,281,609 granted to J. M. Rowell) and SNS devices (U.S. Pat. No. 3,573,661 granted to D. E. McCumber); as well as other devices which do not: a superconducting bridge (Phys. Rev. Let., Vol. 13, p. 195 (1964) by P. W. Anderson et al.) and a point contact structure (U.S. Pat. No. 3,423,607 granted to J. E. Kunzler et al.).
These devices have been proposed for a variety of applications ranging from magnetometers to memory and logic devices. In the latter case, the two basic modes of logic operation, magnetic field switched (MFS) and current-switched (CS), are described in the above-identified Rowell patent. In an MFS device a bias current is set below I.sub.J and a magnetic field is applied to depress I.sub.J below the bias and thereby switch the device from V = 0 to V = 2.DELTA.. In contrast, in a CS device I.sub.J is fixed and a control current is added which exceeds I.sub.J and the device again switches from V = 0 to V = 2.DELTA.. For example, as described at column 2, lines 59 et seq. of Rowell, a pair of control currents can be applied such that only the sum exceeds I.sub.J, thereby performing the AND logic function. In either control current exceeded I.sub.J, however, the OR logic function would be performed.
These basic concepts were extended by W. Anacker et al. (U.S. Pat. No. 3,758,795) to a Josephson MFS logic circuit capable of developing a precisely controlled signal from a Josephson tunneling junction which is used to control (switch) the voltage state of succeeding Josephson junctions. As described at column 2, lines 45 et seq., the circuit comprises a Josephson device connected in parallel with a superconductive transmission line terminated in an impedance designed to prevent reflections. Control means are provided for determining the voltage state of the device; to wit, a bias current source which biases the device below I.sub.J and a plurality of current carrying conductors, the current through which creates a magnetic field which intercepts the Josephson device and alters (depresses) I.sub.J. A second Josephson device is located in proximity to the transmission line. When the first device is switched, the resulting current pulse in the line produces a magnetic field which is coupled to the second device thereby depressing its critical current. The second device then switches. Thus, the change in state of the first device changes the state of the second device and fan-out is provided. Although Anacker et al. allude to the fact that their circuit could be current-switched (column 2, lines 65 et seq.), they caution that this is not an advantageous way of changing the state of a Josephson device.
It is, however, a broad object of my invention to perform memory and logic operations with current-switched Josephson junction devices.
One problem with MFS logic circuits of the Anacker et al. type (one device controlling the state of one or more others) is that relatively large junction sizes (in excess of 2-3 .lambda..sub.J square, where .lambda..sub.J is typically 40 .mu.m) are required in order to get appreciable changes in I.sub.J. Thus, if a superconductive conductor overlays a junction which is only, say, 0.5 .lambda..sub.J square, then 1 mA of control current through the conductor may depress I.sub.J by only 100 .mu.A. This situation would mean that the Josephson devices might have to be biased impractically close I.sub.J for currently available design margins. Alternatively, the control current would have to be increased to say, 5-20 mA, thus wasting additional power.
It is another object of my invention to perform CS operations in Josehson circuits having practical design margins so that the Josephson devices need not be biased inordinately close to I.sub.J, and can operate at current levels well below 1 mA. These lower currents conserve power and make it easier to fabricate matched transmission lines at smaller dimensions.
Another problem with MFS Josephson circuits, especially those operating at currents in the milliampere range, is that the magnetic field used to depress I.sub.J creates well-known resonances in the I-V characteristic.
It is, therefore, yet another object of my invention to provide a CS Josephson logic circuit which operates free of any deleterious resonance effects.